Invisible inks, such as scratch inks, have been utilized on documents for anti-counterfeiting purposes and to create games and other entertainment items such as toys, puzzles and coloring books. A scratch ink includes any ink or material that produces a color change or color initiation by rubbing, scratching, wiping, applying pressure or other heat producing actions. An example of an invisible scratch ink is disclosed in U.S. Pat. No. Re. 36,306, the entire disclosure of which is incorporated herein by reference.
Invisible inks may include a color former leuco dye and an activator that display color upon the application of heat caused by friction associated with rubbing, scratching or wiping of the ink. Such invisible inks are sometimes encapsulated to prevent premature activation of the leuco dye, but the capsules are prone to rupture and cause premature color development during printing, packaging and transit.
In addition, when such invisible inks are produced in larger quantities to cover larger areas of the substrates, the invisible ink may have the undesirable tendency to clump, foam up or increase the probability of premature color development. To address this issue, the activator and leuco dye have been printed separately, one on top of the other, but such two station printing is not ideal because the color development via frictional heat (scratching or rubbing) has a tendency to smear or powder and is more costly to print due to the need for two stations. It has proven difficult to resolve this processing issue by the addition of processing aids since any solubility of the activator or leuco dye can produce premature color activation.